This invention relates to fluidized bed reactors, and more particularly, to a system and method in which a heat exchanger is provided adjacent a fluidized bed reactor.
Fluidized bed reactors generally involve passing air through a bed of particulate material, including a fossil fuel, such as sulfur containing coal, and an adsorbent for the sulfur-oxides generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. When the reactor is utilized in a steam generation system to drive a steam turbine, or the like, water or coolant is passed through conventional water flow circuitry in a heat exchange relation to the fluidized bed material to generate steam. The system includes a separator which separates the entrained particulate solids from the flue gases from the fluidized bed reactor and recycles them into the bed. This results in an attractive combination of high combustion efficiency, high sulfur oxides adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed utilized in the reactor of these type systems is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well defined, or discrete, upper surface. Other types of fluidized beds utilize a "circulating" fluidized bed. According to this technique, the fluidized bed density may be below that of a typical bubbling fluidized bed, the air velocity is equal to or greater than that of a bubbling bed, and the flue gases passing through the bed entrain a substantial amount of the fine particulate solids to the extent that they are substantially saturated therewith.
Also, circulating fluidized beds are characterized by relatively high solids recycling which makes the bed insensitive to fuel heat release patterns, thus minimizing temperature variations, and therefore, stabilizing the nitrogen oxides emissions at a low level. The high solids recycling improves the overall system efficiency owing to the increase in sulfur-oxides adsorbent and fuel residence times which reduces the adsorbent and fuel consumption.
Often in circulating fluidized bed reactors, a heat exchanger is located in the return solids-stream from the cyclone separator which utilizes water cooled surfaces for the extraction of thermal energy at a high heat transfer rate. In steam generation applications this additional thermal energy can be utilized to regulate the exit temperature of the steam to better match the turbine requirements. Typically, at relatively high demand loads, the heat exchanger supplies only a relatively small percentage of the total thermal load to the reactor, while at relatively low demand loads, the heat exchanger could supply up to approximately 20% of the total thermal load.
Unfortunately, while the heat exchanger could thus supply a significant percentage of the total thermal load of a fluidized bed reactor under low demand loads and start-up conditions, the heat exchanger typically has limited capacity for thermal regulation. More particularly, during these low demand loads and start-up conditions, the exit temperature of the water/steam is less than optimum due to the reactor conditions taking precedence. This results in a decrease in the overall efficiency of the system and in an increase in mechanical stress on the external equipment that receives the mismatched coolant.